Semiconductor wafer fabrication processes are especially susceptible to yield loss due to particle contamination. Free particles landing on a wafer surface during processing can produce defects in patterns and deposited layers that result in device failure. A primary source of these free particles is the vacuum processing equipment that is used for process steps such as the deposition of dielectric and conduction layers, the plasma etching of patterns, the electron beam writing of fine patterns, ion implantation of dopants into the wafer surface, and the dry stripping of photoresist.
Such contaminant particles present in the processing equipment range in diameter from less than 100 angstroms to several microns. Typical sources include debris broken from wafers during handling, chemical reactions in the equipment, and the flaking of material from the walls of the equipment. Particles greater in size that one tenth of the smallest pattern geometry are generally of concern as they may produce defects in the wafers that are being fabricated. For example, a 1 megabit dynamic random access memory (DRAM) will use 1 micron lines, and 0.1 micron particles will be of concern, and a 16 megabit DRAM will use 0.5 micron lines, and 0.05 micron particles will be of concern.
The only known means available today to monitor such small particles in vacuum process equipment is to collect them on plates or wafers and scan the collection plates with a scanning electron microscope. This is a very tedious and time consuming procedure, is an off-line measurement, and requires expensive equipment. A real-time, automatic measurement is preferred because it is desirable to detect problems as they occur, so a solution can be effected before significant damage occurs to the wafers being processed.
Real-time techniques exist to monitor particles greater than 0.5 microns in diameter in vacuum, and to monitor extremely small particles much less than 0.5 microns in diameter in air. A real-time system for monitoring particles in vacuum systems is disclosed in copending U.S. patent application Ser. No. 07/041,795, entitled "A Compact Particle Flux Monitor", filed on behalf of P. Borden et al. and assigned to the same assignee. The particle flux monitor employs an optical system to create a net of light in space. As particles pass through this net, they scatter light to photodetectors. This technique is not applicable below 0.5 micron particles size because the particles become small compared to the light wavelength, and the scattering becomes too small to detect. Other techniques for monitoring small particles, such as the airborne particle counter and the condensation nucleation counter (CNC) are not applicable because they use suction to draw air laden with suspended particles into a measuring area. This suction cannot be applied to vacuum systems.
The CNC detects extremely small particles, as small as 0.01 microns in diameter. A description of this counter instrument is provided in the textbook by William C. Hinds, entitled "Aerosol Technology", published by John Wiley and Sons, 1982, see section 13.6. During operation, the instrument passes air through a saturation tube to saturate the air with alcohol vapors. The air is then chilled and is supersaturated thereby forcing the alcohol to condense onto particles suspended in the air, which act as nucleation sites. The particles grow to a size that allows them to be counted using normal light scattering techniques.
Prior art vacuum technology employs vapor booster pumps that use the principles of diffusion and ejection, as described in the textbook entitled "Vacuum Technology", by A. Roth, published by North-Holland Physics Publishing, Second Revised Edition, 1982, see section 5.3. Diffusion pumps operate by creating a flow of the vapors of special oils. Gas molecules are caught in this flow and are carried away, resulting in a pumping action. The vapor flow is supersaturated while retaining the pumping action. Very small particles trapped in this flow will act as nucleation sites for the oil vapors and quickly grow into droplets several microns in diameter. These large droplets can be detected using light scattering.
However the prior art technology and techniques do not provide a means for the detection of extremely small particles in a vacuum or low pressure environments.